The evaluation of spectral distributions has been highly developed over many decades in conjunction with the various spectroscopies which have evolved. In the context of analytic instruments, the positions of spectral lines to be observed is substantially known a priori and it is the relative concentration of the elements present which is to be determined. Positions of expected lines may be perturbed slightly by instrumental effects or by the superposition of proximate lines. At high resolution, line spectra exhibit lineshapes which may be studied by fitting techniques to reveal the quantitative presence of partially overlapping lines.
In the case of certain specific modern spectroscopic instruments the data is photo-electronically accumulated at discrete positions (pixels) with the result that a narrow peak may be defined by only a few datums. Given the discrete nature of the photoreceptive pixels, finite resolution and dispersion of the instrument and possible instability of the instrument to drift, peak position and drift may be determinable as fractional pixel quantities. The spectral evaluation technique of the present invention accommodates this combination of circumstances.
In the particular case where atomic emission spectra are excited from an inductively coupled plasma, the nature of ICP excitation is such that the density of spectral lines is quite high compared to more sedate excitation. Thus, interferences are frequently encountered.
In the prior art, spectral decomposition from ICP-AES has been discussed by van Veen et al., Spectrochimica Acta, v.45B, pp.313 (1990). The method reported there is based upon a Kalman filter technique with the consequence that this prior art procedure is iterative and further requires multiple data acquisitions to examine wavelength shifts. This particular work was accomplished with a scanning type instrument arranged to yield a far greater density of points for peak definition than would ordinarily be available in a broad range instrument with discrete detection elements for the simultaneous acquisition of data over a wide spectral range. Quite apart from the nature of the particular spectroscopic instrument, the spectral treatment as described, is very intensive in computation and time required therefore.
Another approach to spectral decomposition of ICP-AES data is described in U.S. Pat. No. 5,308,982 to Ivaldi, et al. This procedure employs a first (and preferably second) derivative of the spectrum in process to construct a model spectrum which conforms to the observed spectrum by adjustment of appropriate parameters, thereby yielding concentration information. Such methods are practiced at the price of a significantly diminished signal to noise parameter.
In the present work, ICP-AES is carried out in an instrument which is described in U.S. Pat. No. 5,596,407, incorporated by reference herein. This instrument includes an electronic focal plane detector characterized by sequences of discrete pixels arranged in continuous linear arrays.